Near infrared wavelengths (approx. 600-1000 nm) have been used in optical imaging of internal tissues, because near infrared radiation exhibits tissue penetration of up to 15 centimeters. See, e.g., Wyatt, 1997, “Cerebral oxygenation and haemodynamics in the fetus and newborn infant,” Phil. Trans. R. Soc. London B 352:701-706; and Tromberg et al., 1997, “Non-invasive measurements of breast tissue optical properties using frequency-domain photo migration,” Phil. Trans. R. Soc. London B 352:661-667.
Advantages of near infrared imaging over other currently used clinical imaging techniques include the following: potential for simultaneous use of multiple, distinguishable probes (important in molecular imaging); high temporal resolution (important in functional imaging); high spatial resolution (important in in vivo microscopy); and safety (no ionizing radiation).
In near infrared fluorescence imaging, filtered light or a laser with a defined bandwidth is used as a source of excitation light. The excitation light travels through body tissues. When it encounters a near infrared fluorescent molecule (“contrast agent” or “probe”), the excitation light is absorbed. The fluorophore then emits light that has detectably different properties (i.e., spectral properties of the probe (slightly longer wavelength), e.g., fluorescence) from the excitation light.